Speed limits are biology and physics derived. The eye has a max speed, over which it starts to rewrite the history of what you saw. Everytime you have been "frozzen in fear" the first few milliseconds are just the eye rewriting the logs.

So you take that the saccade speed (https://en.wikipedia.org/wiki/Saccade) + speed of visual buffer reaction + reasonable time to break and you get a max speed for that.

Same goes when you have two points of attention, like traffic in front of you and merging traffic, the speed gets reduced to compensate.